Some embodiments relate to semiconductor memory devices and methods of operation thereof and, more particularly, to resistive memory devices and methods of operation thereof.
Semiconductor memory devices for storing data may be classified into volatile memory devices and non-volatile memory devices. The volatile memory devices are typically configured to store data by charging or discharging capacitors in memory cells, and widely used as main memories of various apparatuses. Volatile memory devices, such as Dynamic Random Access Memory (DRAM) and Static Random Access Memory (SRAM) retain stored data while power is supplied and lose the stored data when power is off. Non-volatile memory devices, such as flash memory devices, may maintain stored data even though power is off, and widely used for storing program codes and/or data in computers, mobile devices, etc.
To address demands for high memory capacity, high operation speed and low power consumption, memory devices of various types have been developed that potentially offer high integration density like DRAM, high speed like SRAM and non-volatility like flash memory in the same device. For example, Phase Change Random Access Memory (PRAM) (which uses phase change materials), Resistance Random Access Memory (RRAM) (which uses materials having variable resistance such as transition-metal oxides), and Magnetic Random Access Memory (MRAM) (which uses ferromagnetism materials) are attracting attention as potential next-generation memory devices. The memory storage materials used in such devices share common characteristics, such as resistance that is variable depending on magnitude and/or direction of applied voltage and/or current, with the resistance being maintained without requiring sustained applied voltage and/or current (e.g., refresh is not required).
Memory cells such resistive memory devices may be formed with one resistive element and one switching element so that data may be stored by controlling voltage and/or current of a bitline and a wordline to change resistance of the resistive element. The resistive memory cells may be classified into PRAM, RRAM, MRAM, etc. depending on materials forming the resistive element of the memory cell. For example, the resistive element of a PRAM cell may include phase change materials such as Ge—Sb—Te, resistance of which is varied depending on temperature. The resistive element of a RRAM cell may include a top electrode, a bottom electrode and transition-metal oxides between the top and bottom electrodes. The resistive element of a MRAM cell may include a magnetic top electrode, a magnetic bottom electrode and dielectric materials between the top and bottom electrodes. To potentially reduce chip size, resistive memory devices with stacked structures including resistive memory cell layers stacked on a semiconductor substrate have been developed.